Back in 2008, reports of fires that were caused by overheating batteries in first generation iPod nanos caught the attention of Japan's Ministry of Economy, Trade and Industry (MEI). In turn, MEI ordered Apple to replace defective units. Apple conceded and launched a replacement program in August of 2010. Since then, according to the Mainichi Daily News, Apple has replaced over 5,000 batteries.
The MEI reported that Apple replaced 5,527 batteries in August, up from 232 in July and 62 in June. The jump is attributed to changes Apple made to the program's Web page, which made it easier for customers to find pertinent information.
Apple has stated that the defective batteries overheat only in "very rare cases," but it has not revealed how many of the targeted units -- first generation iPod nanos sold in Japan between September 2005 and September 2006 -- have been sold.
The new iPod nano looks super, but I've always been partial to the design of the original model. It felt like a big Andes Mint. And for the record, mine never got too hot.